Compact source discharge lamp manufacture



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United States Patent 4 Claims ABSTRACT OF THE DISCLOSURE A short are gap quartz arc lamp utilizing pinch sealed inleads in which electrode alignment during sealing is effected by engaging the tip of the cathode in a recess provided in the face of the anode. After sealing, one end of the quartz tube is heated to plasticity and the tube is stretched to disengage the electrodes and accurately set the arc gap, the extent of stretch corresponding to the electrode separation.

This invention relates to the manufacture of discharge lamps requiring accurately aligned electrodes. More specifically, the invention relates to the positioning of electrodes in a compact source (short arc gap) lamp wherein the electrode-inleads are pinch-sealed in the ends of a quartz tube.

An example of a compact source lamp is a high pressure Xenon arc lamp. In such a lamp, an arc is maintained between two electrodes whose distance apart is less than the distance between electrode tips and the tube wall. If proper alignment of electrodes is not obtained, the discharge are may strike to one side of the anode causing inefficient light output and are instability. Accurate alignment of electrodes is particularly necessary in order for the lamp to be suitable for use in optical systems which require a small and precisely placed light source.

In sealing electrode-inleads in quartz arc tubes, pinchsealing is preferred over the older forms of sealing such as vacuum sealing or graded seals. In pinch-sealing, a foil type electrode-inlead assembly is used and sealing is done by pressing or pinching the walls of the heat-softened vitreous tube between a pair of opposed jaws which are moved in a direction perpendicular to the plane of the foil to press or pinch the quartz fiat about it. A foil or thin, flat foliated portion in the electrode-inlead assembly is necessary to assure a vacuum tight seal. However, the foil weakens the structure and causes difficulty in electrode alignment.

Various solutions to the problem of maintaining electrode alignment during pinch-sealing have been proposed. One solution is found in Patent 3,151,922, Preschel et al., wherein two electrode-inlead assemblies are connected by a knock-out portion which joins the two electrodes. After pinch-sealing both ends, the knock-out portion is tapped out from between the two electrodes by a rod inserted through the exhaust tube. The knock-out portion defines the arc gap, and the exhaust tube is, of necessity, located in line with the arc gap. Hence, after tip-off of the exhaust tube, a scar remains in a critical location which could render the lamp unacceptable for optical use. Also, this method is not suitable for D-C lamps in which the anode is much larger than the cathode. Therefore, objects of this invention are to provide a lamp construction and manufacturing method maintaining electrode alignment in pinch-sealing while avoiding the foregoing drawbacks.

Another solution to the problem is found in pending application Ser. No. 262,009, Wilson et al., filed Mar. 1,

Patented Jan. 7, 1969 ice 1963, now Patent No. 3,250,941, and assigned to the same assignee as the present application, wherein a coil spring wound around an electrode bears against the walls of the tube to maintain electrode alignment. Using this method, the first electrode is held by the coil to align it during pinch-sealing, and the coil is then removed from around the electrode via the open end. Next a coil is placed around the second electrode, and the second assembly is pinch-sealed. However, the coil must be left in the discharge tube after the second pinch-seal is completed since both ends of the tube are now sealed. This method leaves an extra component inside the envelope and is restricted to lamps where the second electrode is massive enough to shield the coil so that it does not interfere with the are or with the optics of the lamp. Therefore, a further object of the invention is to provide a lamp construction and method of manufacture not restricted by the size of the electrodes and eliminating all unnecessary accessory components in the finished lamp.

Briefly stated, by way of example of the invention, alignment of the first electrode-inlead assembly is maintained during pinch-sealing by external means which penetrate the vitreous tube through the open end and engage the electrode. Alignment of the second electrodeinlead assembly is maintained during pinch-sealing by engaging the tip of one electrode in a recess provided in the face of the other. The recess and tip may have any geometrical shape favoring interengagement. Hence, if the tip of one electrode is a cone, the recess in the other is preferably conical.

After completion of the pinch seals, one rounded end of the tube or envelope is heated to plasticity, and the tube is stretched to disengage and set the arc gap. The heated end of the tube is deformed out of round and necked down to an extent corresponding to the axial disengagement of recess and tip of the electrodes. However, since the stretching of the tube moves the electrode axially only, the alignment, achieved while the electrodes were interengaged, remains undisturbed.

Further objects and advantages of the invention will appear from the following detailed description to be read in conjunction with the accompanying drawing wherein like reference symbols denote corresponding parts throughout the several views.

In the drawing:

FIG. 1 is a side view of a compact source lamp embodying the invention.

FIG. 1a is a cross-section view along the line Z-Z of FIG. 1 showing the recess in the anode.

FIG. 2 illustrates in cross-section the quartz tube and the anode electrode-inlead assembly mounted on the spindle and maintained in alignment by external means prior to first pinch-sealing.

FIG. 2a is a similar view showing the jaws closed on the tube end during the first pinch-sealing.

FIG. 3 illustrates the first electrode engaging the second electrode previous to the second pinch-sealing.

FIG. 3a illustrates the envelope after the second pinchsealing.

FIG. 4 illustrates the heating of one end of the envelope to plasticity previous to stretching.

FIG. 4a shows the envelope after stretching has set the arc gap between the electrodes,

Referring to the drawing, particularly FIG. 1, the illustrated compact source lamp 10 includes an arc tube 11 which is suitably made of quartz or quartz-like glass. The tube 11 includes a bulbous or globular center section 12 which increases the distance between the electrode tips and the tube wall which keeps the wall temperature down. Also, this decreases the probability of the are striking the envelope and following its contour to the 3 other electrode. The ends of tube 11 are sealed by full diameter pinch seals 13 and 14 each one of which holds an electrode-inlead assembly.

The electrode-inlead assemblies comprise external inleads 15 and 16, folds 17 and 18, internal inleads 19 and 20 and electrodes consisting of anode 21 and cathode 22. The internal inleads 19 and 20 are preferably made from tungsten wire, and electrodes 21 and 22 are preferably shaped from tungsten rod. The internal inleads 19 and 20 are inserted in to previously made holes in the outer ends of electrodes 21 and 22 and platinum-brazed in place. The other ends of the tungsten wire inleads 19 and 20 which have been flattened are welded to foils 17 and 18 which are preferably of molybdenum. The weld 1s usually performed with a platinum tab spaced between the internal inleads and the foils. Similarly, external inleads 15 and 16 preferably made of molybdenum wire have their flattened ends Welded to the other ends of c1 ils 17 and 18 to complete the electrode-inlead assem- 1es.

Anode 21, the more massive of the two electrodes, is cylindrical in shape with a flat front end face containing a generally conical recess 24 as shown in FIG. la. Cathode 22, the smaller electrode, is generally cylindrical with a conical tip 25. The electrodes are designed to suit the discharge; different geometrical shapes could be used for the recess in one intended to accommodate the tip of the other provided interengagement can readily be achieved.

Inasmuch as the first electrode is used after pinch-sealing to maintain the alignment of the second and anode 21 is the more massive, the anode-inlead assembly is preferably pinch-sealed first. The anode-inlead assembly is vertically mounted on spindle 26 by inserting external inlead 15 into well 27 in the end of the spindle while quartz tube 11 is placed around it, as shown in FIG. 2. The rigidity of foil 17 is insufficient to maintain anode 21 on the axis of the envelope and external means penetrating into the tube through its open end are used for this purpose prior to and during pinch-sealing. The external means comprise rod 28 which carries anode holder 29 shaped like an inverted cup to accommodate anode 21. The rod passes through hole 35 in stopper 31 which is pressed into the open end of the tube; this assures alignment of the rod on the tube axis.

During the heating of tube 11 to plasticity (FIG. 2), nitrogen is pumped through the exhaust tube opening and around the spindle as indicated by arrows A and B. The nitrogen atmosphere prevents the heat from burners 32 from oxidizing the anode-inlead assembly.

After the quartz tube has been heated to plasticity, burners 32 are withdrawn and pinching jaws 33 press the quartz about the anode-inlead assembly (see FIG. 2a). In the absence of additional support, foil portion 17 may readily become distorted during pinching which would cause misalignment of anode 21. However rod 28 and holder 29 maintain the alignment of the anode during pinching whereupon foil portion 17 and inlead 19 become fixed in the quartz. The inner faces of the pinching jaws 33 are shaped to press the quartz flat over foil portion 17 but are relieved to thicken the quartz in regions 34 and 35 about inleads 15 and 19. The flat pinch assures hermetic sealing of the foil while the thickened regions give added strength for anchoring the inleads. More details on a suitable pinch sealing method and apparatus are given in Patent 2,965,698, Gottschalk.

After the quartz has cooled sufiiciently, tube 11 which now has anode 21 sealed into one end is inverted from its initial position and placed over the cathode-inlead assembly vertically mounted on spindle 26 as shown in FIG. 3. Tube 11 is lowered and adjusted as needed to engage cathode tip in recess 24 of anode 21. Interengagement of the electrodes aligns cathode 22 with anode 21 even should anode 21 be slightly off the tube axis consequent upon the first pinch seal. Since foil 18 is flexible, it can bend slightly to compensate for any misalignment of anode 21. A protective nitrogen atmosphere is again supplied while the flames from burners 32 heat the quartz tube in the area of the cathode-inlead assembly to plasticity.

Once the quartz is softened, pinching jaws 33 perform the same pinching action previously described (see FIG. 3a). During pinching, in accordance with the invention, the cathode-inlead assembly is maintained in alignment by the engagement of cathode tip 25 in anode recess 24 while the assembly is supported from the other end by the engagement of inlead 16 in well 27 Within spindle 26. As a result, pinching produces perfectly aligned electrodes, as shown in FIG. 3a.

After the second pinch-sealing, the arc gap is opened up and set by axial displacement of one electrode. Tube 11 may be heated to plasticity at either end but preferably at the cathode end soon after the second pinch-seal while the quartz is still hot. The flames from burner 32 are directed against the rounded end 36 of the arc chamber. The lower end of the tube is fixed by tightening screw 37 which locks external inlead 16 in spindle 26, while external inlead 15 is gripped by jaws 38 to stretch the tube a predetermined vertical distance with corresponding necking-down in region 36. The necking down in the region of end protuberance 36 represents an elongation relative to the rounded protuberance at the other end which corresponds to the axial separation of recess and tip of the electrodes measured from inter-engagement, as may be seen in FIGS. 4 and 4a of the drawing. The electrodes are thus disengaged and the interelectrode spacing or arc gap 39 is accurately set. Practical considerations such as the extent of permissible reduction in the thickness of the quartz in region 36 of the tube place a limit on the size of the arc gap which can set in this manner.

To complete the manufacture of the lamp, it is outgassed and an ionizable filling such as xenon is pumped into the envelope through exhaust tube 40 which is then tipped off leaving a scar 40a.

Our invention thus provides a lamp having both precise alignment of the electrodes and an accurately set arc gap between the electrodes. The precise alignment of the electrodes is predicated on the feature of the recess in the face of one electrode accommodating the tip of the other whereby interengagement of the electrodes is had during pinch-sealing. The proper arc gap setting is then acquired by stretching the tube axially so that the alignment is not disturbed thereby. A reduction in wall thickness corresponding to the gap distance occurs in the envelope region which is stretched and necked down. However, by choosing an envelope having a wall thickness in that region adequate to sustain the reduction which takes place, a reliable product is achieved having the desired qualities.

The specific embodiment of the invention which has been illustrated and described in detail is intended by way of example and the scope of the invention is to be determined by the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making a compact source discharge lamp of the kind utilizing pinch seals with foil type electrode-inlead assemblies at opposite ends of a vitreous tubular envelope and requiring accurate electrode alignment and spacing comprising:

providing a recess in the front face of one electrode adapted to accommodate the tip of the other, pinch-sealing one electrode into one end of the vitreous tube,

locating the other electrode inlead assembly on the tube axis by interengaging the tip of one electrode with the recess in the face of the other,

pinch-sealing the second electrode into the tube,

heating one end of the vitreous tube to plasticity,

and axially stretching the envelope a predetermined amount in order to disengage the electrodes and set the gap between them. 1 2. The method of making a compact source discharge lamp defined in claim 1 wherein one of the electrodes is a relativelynnassive anode and the other is a smaller cathode and the anode-inlead assembly is pinch-sealed first. 'l

3. The method of making a compact source discharge lamp defined. in claim 2 wherein the first electrode is maintained in alignment during pinch-sealing by' reaching into the tube through its open end with external means. 4. The method of making a compact source discharge lamp defined in claim 2 wherein the massive anode has a References Cited UNITED STATES PATENTS 5/1959 Haidinger 313--216 X 7/1959 Anderson 313-217 X 8/1960 Holmes 313209 12/1960 Ittig et a1. 313--217 X 12/1963 Story 313-217 X [0 JAMES w. LAWRENCE, Primary Examiner. PALMER C. DEMEO, Assistant Examiner.

US Cl. X.R.

flat front end face and it is provided with a recess accom- 15 313-214, 216, 220; 65-59 modating the cathode which has a conical tip. 

